There are several types of power losses in switching power converters. To illustrate this, one particular type of switching converter is described briefly here. It is a DC-to-AC switching converter typically called an “inverter”. An inverter receives a DC supply voltage and outputs a sinusoidal AC voltage or current. There are various circuit topologies for inverters, but FIG. 1A illustrates one example of part of one exemplary inverter circuit. The inverter circuit involves a so-called “high-side” transistor designated QHS and a so-called “low-side” transistor designated QLS. Each of these transistors is an N-channel field effect transistor which is sometimes colloquially called an N-channel MOSFET (Metal-Oxide Semiconductor Field Effect Transistor). Each of these transistors is realized as part of a semiconductor die. There is an inherent body diode that is a part of that die. The diode may be illustrated in the symbol of the N-channel transistor, or it may not be illustrated at all, but it is present along with the transistor. In the inverter circuit, there is a first DC supply voltage present on node N1, and a second higher DC supply voltage present on node N2. Node GND is a ground node. The reference numeral L identifies the first winding (primary-side winding) of a transformer. The core of the transformer and the second winding (secondary-side winding) of the transformer are not shown. The overall purpose of the inverter circuit is to generate an AC current flow through the first winding L. This causes a similar AC current to flow in the second winding of the transformer, and this AC current in the second winding is made to pass through a load. The control and drive circuitry that controls the high-side and low-side transistors is not shown.
In a first half cycle of the output sinusoidal AC current flowing in the winding L, the high-side transistor is controlled to be off. This is designated in FIG. 1A, FIG. 1B, FIG. 1C and FIG. 1D by the text “OFF” that appears by the high-side transistor QHS. The low-side transistor QLS, on the other hand, is switched on and off in such a way as to cause the sinusoidal AC current to flow through the first winding. Then in a second half cycle of the sinusoidal AC current flow, the low-side transistor QLS is controlled to be off. Operation of the inverter circuit in this second half cycle is not illustrated. In the second half cycle of the sinusoidal AC current flow, the high-side transistor QHS is switched on and off in such a way as to cause the sinusoidal AC current to flow.
FIG. 1A, FIG. 1B, FIG. 1C and FIG. 1D illustrate current flows during an exemplary first half cycle of the sinusoidal AC current. FIG. 1A illustrates a first situation. The low-side transistor QLS is controlled to be on. Current is made to flow as illustrated by arrow A. The current flows from node N1, through the winding L, through the transistor QLS, and to the ground node GND. After a period of time, the low-side transistor QLS is turned off. This gives rise to the situation illustrated in FIG. 1B. Since the current cannot stop instantaneously in the inductance of the first winding L, and because it also cannot flow through the blocking low-side transistor QLS, it flows in the path illustrated by arrow B. The high-side transistor QHS is off, but the current B flows through the body diode DHS up to node N2. After an amount of time, the low-side transistor QLS is turned on again. Current then flows as illustrated by arrow C in FIG. 1C. The low-side transistor QLS is on and conductive, so current flows from the node N1, through the winding L, through the low-side transistor QLS, and to the ground node GND. When the low-side transistor QLS first turned on, however, there is a reverse voltage applied across the body diode DHS of the high-side transistor. This causes a short burst of reverse recovery current to flow through the body diode DHS. This burst of reverse recovery current flows in the path C illustrated in FIG. 1C. Once this reverse recovery current flow has stopped, then the current flow is as illustrated in FIG. 1D.
Current flow through body diode DHS can cause power losses in the switching converter. The surge of reverse recovery current illustrated in FIG. 1C, although of relatively short duration, is a large current and it occurs during a time when a large reverse voltage is present across the body diode. The integration over time of the instantaneous current flow through the body diode DHS multiplied by the instantaneous voltage drop across the body diode DHS represents energy loss. This is energy loss due to the flow of the reverse recovery current. In addition, there is an energy loss due to forward current flow through the body diode DHS. When the current B illustrated in FIG. 1B flows across the body diode DHS, there is about a one volt voltage drop across the body diode DHS. The integration of the instantaneous current flow through the body diode DHS multiplied by the instantaneous voltage drop across the body diode DHS represents energy loss.